The amplification and line driving of data signals for transmission, such as for use in digital subscriber lines (DSL), constitutes an important link in the transmission of data signals. The transmitted data signals will probably have a substantially Gaussian distribution: a given percentage of symbols constituting the data signals will have an amplitude below a certain amount, and the rest will have an amplitude above that certain amount. When amplifying these symbols for transmission by a line driver, one has typically set the power supply rail of the line driver amplifier at a large voltage level so that there is a low probability of clipping. However, the large voltage level is still high enough to lead to excessive power consumption, because much of the time the line driver will not be amplifying a signal requiring the large voltage level.
One attempted solution to this problem is the creation and employment of a Class G amplifier. A Class G amplifier may generally be defined as an amplifier, which has associated with it two sub-amplifiers with different operating voltages. In one prior-art embodiment, the Class G amplifier employs two sub-amplifiers coupled in parallel, in which the Class G amplifier's analog circuitry decides which sub-amplifier to use to amplify the data signal. The decision is based upon the amplitude of the data signal, thereby hopefully leading to a power savings while minimizing the odds of clipping data signals.
While prior art zero overhead Class G amplifiers generally provide improved performance, their performance still involves the possibility of clipping the data signal (because the low operating voltage sub-amplifier is used when the incoming signal is above the threshold) or consuming excessive power (because the high operating voltage sub-amplifier is used when the incoming signal is below the threshold). These inefficiencies can arise because of an inherent inaccuracy in making the threshold determination after the incoming signal has been converted to analog domain and also from the fact that the signal may be distorted by analog filters subsequent to the threshold determination.
For instance, filters may have associated with them a phenomenon known as “group delay distortion”. In group delay distortion, different frequencies traversing a set of filters may have different propagation constants due to characteristics associated with the filters. As the various sinusoidal signals are delayed, they may constructively or destructively interfere, yielding an output signal different from the input signal. Constructive interference may cause the zero-overhead amplifier to clip a data signal, as the sub-amplifier was configured for signal data with a lower amplitude. Alternatively, destructive interference may cause the zero-overhead amplifier to waste power, as the sub-amplifier may be needlessly coupled to a rail to accommodate a now-attenuated data signal. These and other problems lead to difficulties in determining the correct triggering threshold for the peak detector circuit when making the determination of the triggering threshold in the digital domain.
Therefore what is needed in the art is a zero-overhead amplifier that overcomes the above-described and other limitations of the prior art.